The present invention relates to ink jet printing, and in particular to an improved system and method for auto-threshold adjustment for phasing in an ink jet printer.
Continuous ink jet printers are well known in the field of industrial coding and marking, and are widely used for printing information, such as expiry dates, on various types of substrate passing the printer on production lines. As shown in FIG. 1, a jet of ink is broken up into a regular stream of uniform ink drops by an oscillating piezoelectric element. The drops then pass a charging plate, which charges individual drops at a selected voltage. The drops then pass through a transverse electric field provided across a pair of deflection plates. Each drop is deflected by an amount which depends on its charge. If the drop is uncharged, it will pass through the deflection plates without deflection. Uncharged and slightly charged drops are collected in a catcher and returned to the ink supply for reuse. A drop following a trajectory that misses the catcher will impinge on the substrate at a point determined by the charge on the drop. Often, each charged drop is interspersed by a guard drop with substantially no charge to decrease electrostatic and aerodynamic interaction between charged drops. As the substrate is moving past the printer, the placement of the drop on the substrate in the direction of motion of the substrate will have a component determined by the time at which the drop is released. The direction of motion of the substrate will hereinafter be referred to as the horizontal direction, and the direction perpendicular to this, in the plane of the substrate will hereinafter be referred to as the vertical direction. These directions are unrelated to the orientation of the substrate and printer in space. If the drops are deflected vertically, the placement of a drop in the vertical and horizontal direction is determined both by the charge on the drop and the position of the substrate.
It is general practice to provide predefined raster patterns, with the matrix for each pattern, customarily representing a character, of a predetermined size. For example, a 5 high by 5 wide matrix representing an image, as shown in FIG. 2, can be created which represents a whole image such as a character or a portion of an image. FIG. 3 shows a 7 by 9 matrix which yields better defined characters. A technique which has become widely used for printing these characters or portions of images is disclosed in U.S. Pat. No. 3,298,030 (Lewis et al). A stroke is defined for each column of the matrix and represents a slice of the image. Each usable drop is assigned to each pixel (dot position) in the stroke. If the pixel is a blank pixel, then the drop is not charged and is captured by the catcher to be sent back to the ink supply. If the pixel is to be printed, an appropriate charge is put on the drop so that it is deflected to follow a trajectory that intercepts the substrate at the appropriate position in the column for that stroke. This cycle repeats for all strokes in a character and then starts again for the next character. If the drops are deflected transversely to the direction of travel of the substrate, a set of drops forming a stroke will clearly lie along a diagonal line, as the substrate will move a certain distance between each drop in the stroke. The angular deviation of the line from vertical will increase with the speed of the substrate relative to the drop emission rate. This angular deviation can be counteracted by angling the deflection plates away from the vertical direction by an amount dependent on the expected speed of the substrate. If drops in a stroke are not sequentially allocated to equally spaced positions on the substrate, the points will no longer lie along a straight line.
In order to maintain a simple matrix raster pattern, with straight lines in any direction in the matrix mapping onto straight lines on the substrate, it is necessary to print drops in a stroke sequentially with an equal time interval between each stroke. A stroke takes the same time whether it contains one printed drop or five printed drops. Generally, a varying number of extra guard drops are used at the end of each stroke to permit variation in the substrate speed on a stroke by stroke basis.
Current systems may generate feedback for ink drop charging by measuring peaks of a charge signal obtained from an ink stream. Systems often employ a threshold against which to compare the charge signal. In current systems, the threshold is physically set at a certain trip point, and anything above that trip point is a “good” phase. Sensors used to obtain charge signal data for an ink stream are placed in an ink catcher and are often physically adjusted to acquire data. This process is inconvenient, time-consuming, and inaccurate for printer users. System configurations and thresholds must be changed manually for different inks, printers, and environments. Additionally, positioning a sensor in the ink catcher introduces additional interference in charge measurements due to the catcher and distance from a nozzle originating the ink stream. Furthermore, it is difficult to isolate individual ink drops in the ink catcher. Thus, there is a need for an improved system and method for measuring charge and setting thresholds in an ink printing system.
There is a need for an improved system and method for auto-threshold adjustment for phasing in an ink jet printer.